10-2: RNA and 10-3: Protein Synthesis

Objectives: 10-2• Explain the primary functions of RNA• Compare the structure of RNA with that of DNA• Describe the structure and function of each type of DNA• Summarize the process of transcription

Objectives: 10-3• Describe the genetic code• Distinguish between a codon and an anticodon, and state where each is

found• Explain the roles of the start and stop codon• Summarize the process of translation

12–3 RNA and Protein Synthesis

•Genes are coded DNA instructions that control the production of proteins. Proteins are chains of Amino Acids.•Protein builds, maintains, and replaces the tissues in your body. Your muscles, your organs, and your immune system are made up mostly of protein.•Genetic messages can be decoded by copying part of the nucleotide sequence from DNA into RNA. •RNA contains coded information for making proteins.

The Structure of RNA

•The Structure of RNA• RNA consists of a long chain of nucleotides.• *Each nucleotide is made up of a 5-carbon sugar,

a phosphate group, and a nitrogenous base.

The Structure of RNA

•*There are three main differences between RNA and DNA:

• The sugar in RNA is ribose instead of deoxyribose. • RNA is generally single-stranded.• RNA contains uracil in place of thymine.

Types of RNA

–What are the three main types of RNA?

Types of RNA

• There are three main types of RNA:– messenger RNA (mRNA)

– ribosomal RNA (rRNA)

– transfer RNA (tRNA)

Types of RNA

•*Messenger RNA (mRNA) carries copies of instructions for assembling amino acids into proteins.

Types of RNA

•*Ribosomes are made up of proteins and ribosomal RNA (rRNA).

Ribosome

Ribosomal RNA

Types of RNA

•*During protein construction, transfer RNA (tRNA) transfers each amino acid to the ribosome.

Amino acid

Transfer RNA

Transcription

–What is transcription?

Transcription

•Transcription• *RNA molecules are produced by copying part of a

nucleotide sequence of DNA into a complementary sequence in RNA. This process is called transcription.

• *Transcription requires the enzyme RNA polymerase.

Transcription•*During transcription, RNA polymerase binds to DNA and separates the DNA strands. •*RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA.

Transcription

•RNA polymerase binds only to regions of DNA known as promoters.•*Promoters are signals in DNA that indicate to the enzyme where to bind to make RNA.

Transcription

RNA

RNA polymerase

DNA

RNA Editing

• *The DNA of eukaryotic genes contains sequences of nucleotides, called introns, that are not involved in coding for proteins.

• *The DNA sequences that code for proteins are called exons.

• *When RNA molecules are formed, introns and exons are copied from DNA.

RNA Editing•The introns are cut out of RNA molecules.

•The exons are the spliced together to form mRNA.

Exon IntronDNA

Pre-mRNA

mRNA

Cap Tail

The Genetic Code

• The genetic code is the “language” of mRNA instructions.

• *The code is written using four “letters” (the bases: A, U, C, and G).

The Genetic Code

•*A codon consists of three consecutive nucleotides on mRNA that specify a particular amino acid.

The Genetic Code• *Each codon specifies a particular amino

acid that is to be placed on the polypeptide chain.

• *Some amino acids can be specified by more than one codon.

The Genetic Code

The Genetic Code• *There is one codon AUG that can either

specify the amino acid methionine or serve as a “start” codon for protein synthesis.

• *There are three “stop” codons that do not code for any amino acid. These “stop” codons signify the end of a polypeptide.

Translation

•What is translation?

Translation•*Translation is the decoding of an mRNA message into a polypeptide chain (protein).

•*Translation takes place on ribosomes.

•*During translation, the cell uses information from messenger RNA to produce proteins.

Translation

Nucleus

mRNA

•*Messenger RNA is transcribed in the nucleus, and then enters the cytoplasm where it attaches to a ribosome.

Translation

•*Translation begins when an mRNA molecule attaches to a ribosome.•*As each codon of the mRNA molecule moves through the ribosome, the proper amino acid is brought into the ribosome by tRNA.•*In the ribosome, the amino acid is transferred to the growing polypeptide chain.

Translation

•*Each tRNA molecule carries only one kind of amino acid. •*In addition to an amino acid, each tRNA molecule has three unpaired bases. •*These bases, called the anticodon, are complementary to one mRNA codon.

Translation

LysinetRNAPhenylalanine

Methionine

Ribosome

mRNAStart codon

•The ribosome binds new tRNA molecules and amino acids as it moves along the mRNA.

Translation

•Protein Synthesis tRNA

Ribosome

mRNA

Lysine

Translation direction

Translation•The process continues until the ribosome reaches a stop codon.

Polypeptide

Ribosome

tRNA

mRNA

The Roles of RNA and DNA

• *The cell uses the DNA “master plan” to prepare RNA “blueprints.” The DNA stays in the nucleus.

• *The RNA molecules go to the protein building sites in the cytoplasm—the ribosomes.

Genes and Proteins

• Genes contain instructions for assembling proteins.

• Many proteins are enzymes, which catalyze and regulate chemical reactions.

• Proteins are each specifically designed to build or operate a component of a living cell.

•The sequence of bases in DNA is used as a template for mRNA. •The codons of mRNA specify the sequence of amino acids in a protein.

CodonCodon Codon

Codon Codon Codon

mRNA

Alanine Arginine Leucine

Amino acids within a polypeptide

Single strand of DNA